Differential pressure sensors measure a difference in pressure between two isolated fluids or gasses. When used in an environment which includes conductive or corrosive gasses or fluids, the sensor must be isolated from these harsh media in order to protect the sensor itself, as well as the electronic or electrical components attached thereto. Differential pressure sensors are harder to isolate from harsh media than gage or absolute pressure sensors due to the presence of two pressure sources being applied to opposing sides of the sensor. Therefore, both sides of the sensor must be isolated in some way or the sensor may be damaged.
A differential pressure sensor (or transducer) converts a difference in pressure to an electrical signal that can be measured to determine the differential pressure value. A pressure-sensing device of the sensor is typically manufactured using micro-machined or Micro-Electro-Mechanical System (MEMS) type methods. This technology is used to manufacture commercial semiconductors along with etching and bonding techniques to fabricate very small, inexpensive devices that convert differential pressure to an electrical signal. The materials used in these devices do not resist corrosion as well as other well-known corrosive resistant metals such as stainless steel, titanium, copper and brass, which are typically used in corrosive fluid and gas plumbing. For this reason, an isolation method is required to act as a barrier for corrosion but allow pressure to be communicated to the pressure-sensing device without substantially degrading a generated signal.
The pressure-sensing die is formed from a semiconductor material such as silicon. The die is formed from a silicon wafer by methods such as dicing to produce a silicon structure, which is thinned to create a cavity and define an associated diaphragm. Piezoresistive elements are formed or placed at the surface of the diaphragm and are configured to exhibit resistance that is proportional to the strain placed on the thinned semiconductor material forming the diaphragm.
Differential pressure sensors may be used to measure pressure relating to harsh media, for example, petroleum fluids, acids, and the like. The semiconductor surfaces of the pressure-sensing die, which support the piezoresistive elements and their associated electrical connections, may be damaged or their lives shortened by exposure to harsh media. To allow use of semiconductor pressure-sensing dies in harsh environments, the pressure-sensing die may be inserted in a package which protects the pressure-sensing die from the media whose pressure is being measured.
A suitable package may include one or more input ports for receiving the media whose pressure is to be measured. The media being measured enters a first volume of the package which is isolated from the pressure-sensing die by a flexible membrane or diaphragm. The flexible membrane or diaphragm is formed from a material which can withstand any harsh effects related to the media whose pressure is being measured. For example, the flexible membrane or diaphragm that contacts the measured media may comprise stainless steel. The flexible membrane or diaphragm is configured to flex when the pressure of the media under test exerts a force to the surface of the flexible membrane or diaphragm.
The package may further define a second volume in contact with the flexible membrane or diaphragm opposite the first volume containing the media under test. The second volume is further in fluid contact with a pressure sensitive surface of the pressure-sensing diaphragm defined in the pressure-sensing die. The second volume is filled with a fluid that may safely contact the diaphragm and associated electrical connections of the pressure-sensing die. For example, the second volume may be filled with silicon oil, which will not harm the semiconductor-based pressure-sensing die. When the media under test is under pressure, it fills the first volume of the package and exerts a force on the flexible membrane, which flexes under the applied force. The flexing of the flexible membrane or diaphragm, in turn, exerts a force against the fluid contained in the second volume. This force, which is representative of the pressure of the media under test, is transferred through the fluid in the second volume to the semiconductor diaphragm of the pressure-sensing die. The applied force causes flexing of the semiconductor diaphragm and the piezoresistive elements formed on the semiconductor diaphragm's surface. The strain on the piezoresistive elements cause their resistance to vary in proportion to the applied force due to the media pressure. An electrical circuit connected to the piezoresistive elements creates an electrical signal, based in part on the resistance value of the piezoresistive elements. Accordingly, the electrical signal is representative of the pressure of the media under test.
Packaging of the pressure-sensing die increases the complexity of manufacturing pressure sensors which may be used in harsh environments. Furthermore, the package may have to be fabricated to adapt to a specific use. Therefore, pressure-sensing dies, along with their associated circuitry, are desired which can be easily fabricated and adapted for installation into a variety of packaging implementations.